The invention relates to a method for mounting a position measuring system on a first part and on a second part movable relative to said first part, wherein said position measuring system comprises a gauge, which has a pitch carrier and is fixable on said first part, and a sensing head fixable on said second part, said method comprising the steps of fixing the sensing head on the gauge such that the sensing head and the gauge are fixed in position relative to each other, fixing the gauge on the first part and the sensing head on the second part, and releasing said fixation between the sensing head and the gauge. The invention further relates to a position measuring system for sensing a movement between a first part and a second part, which is movable relative to the first part, wherein the position measuring system comprises a gauge, which is fixable on the first part, and a pitch carrier, a sensing head, which is fixable on the second part and senses a movement of the gauge relative to the sensing head, and a fixation by which said gauge and said sensing head can be fixed in a predetermined position relative to each other.
Such devices are needed to measure the rotational position or the linear displacement of two objects which are movable relative to each other. The gauge moving relative to the sensing unit is sensed using a sensing unit, and the sensed information is transformed into position information by means of an evaluating unit. In doing so, the most diverse physical sensing principles are applied. Preferred principles are photoelectric, magnetic, inductive or capacitative sensing methods. However, as a basic principle, all methods have in common that the sensing head has to be positioned within predetermined, narrow tolerances of position and angle over the entire range of measurement, depending on the resolution and precision to be achieved.
A pre-mounted angle-measuring device without its own support is known from U.S. Pat. No. 4,639,595. The pitch carrier with its pitch is fixed on and adjusted on a hub as a gauge. In the pre-mounted state, a clamping spring holds the sensing device and the hub together. A fixed stop is provided on the hub, said fixed stop contacting the sensing device during pre-assembly and fixing the hub in the axial and radial directions. In this condition, the angle-measuring device is slid onto the driving shaft, and the sensing device is fixed on the mounting surface of the drive unit. The clamping spring is then released from its engagement, and the hub is axially displaced on the driving shaft. Said displacement is required in order to undo the contact of the stop of the hub and the sensing unit and to ensure frictionless relative movement between the hub and the sensing device.
Although this angle-measuring device, in a pre-assembled state, allows a defined alignment between the contacting surfaces of the hub and of the sensing device in the radial and axial directions, said alignment must be undone during assembly by axially displacing the hub, so that no defined alignment, neither in the radial nor in the axial directions, can subsequently be achieved without further auxiliary means. A further great deficiency of this arrangement consists in that the sensing device is preadjusted only relative to a stop on the hub, but the actual functional alignment is required directly between the sensing unit and the sensing structures on the pitch carrier. In many applications, the use of an additional hub or of other intermediate parts for assembly of the gauge is not possible. These intermediate parts deteriorate the precision of adjustments by considerable cumulative tolerances and, in many cases, also lead to expensive solutions.
Another solution concerning the assembly of preadjusted pitch disks for the sensing device is described in the published patent application DE 4,304,914 A1. Using a mounting device, the alignment of the axial and radial positions of the sensing device and of the pitch disk are achieved via a hub comprising a groove having specific tolerances and via corresponding receiving holes in the stator. In order to transmit this association of positions, the adjusted unit is fitted over the driving shaft, and the stator is then fixed, for example, on the flange of a motor, whereupon the hub, together with the pitch disk, is fixed on the driving shaft.
This solution also uses a complicated and expensive hub. This solution has the disadvantage of the considerable cumulative mechanical tolerances, which, on the one hand, require expensive manufacturing processes, and, on the other hand, the achievable tolerance of adjustment is very hard to achieve, in particular for high precision requirements. For example, the tolerances on fit between the internal hub diameter and the receiving shaft should be selected to be very narrow, so as to achieve small errors of eccentricity of the pitch on the pitch disk, which, however, complicates the process of mounting during fitting of the adjusted unit. Also, the fixing surface of the stator in its position relative to the driving shaft should be within narrow tolerances to avoid generating stresses between the stator and the pitch disk when fixing the stator, which stresses would lead to radial and axial displacements between the stator and the pitch upon release of the mounting device.
DE 3,740,477 A1 describes a method wherein the sensing head is fixed on a hub of an angle-measuring disk. A desired angle between the sensing head and the angle-measuring disk may be set accordingly.
These aforementioned solutions relate, above all, to the field of application of built-in rotation encoders.
In view thereof, it is the object of the invention to improve a method and an arrangement for mounting a sensing head of a gauge—preferably for rotational and linear measuring systems—of the aforementioned type such that inexpensive manufacture and easy and quick mounting in encoder systems or in the case of system integrations is possible.